As integrated circuits in semiconductor devices decrease in size, the photomasks used to fabricate such integrated circuits must permit formation of these smaller and smaller integrated circuits and devices. Attenuated phase-shifting masks (APSM) are one type of photomask that have been developed to allow fabrication of microminiaturized integrated circuits by lithography processes. APSMs have a mask pattern, formed on a transparent substrate, comprising a light, or radiation, transmittable portion through which a light is transmitted that has such an intensity as to substantially contribute to the exposure, and a light semi-transmittable portion, generally comprised of a shifter layer, through which a light is transmitted that has such an intensity as not to substantially contribute to the exposure. APSMs are constituted so as to substantially invert to the phase of the light passed through the light semi-transmittable portion so that the light passed through the neighborhood of a boundary between the light transmittable portion and the light semi-transmittable portion can be mutually offset and the contrast of the boundary can be successfully maintained.
This destructive interference at the edges of the patterned shifter layer provide for sharper edged devices to be formed on the semiconductor structure that are vital for microminuatized integrated circuits. The shifter layer is only partially transmissive to light because if the transmitted radiation is too intense, it would expose a photoresist layer making patterning difficult.
Due to bad chemical durability, only 6% transmittance shifters are commercially available for deep ultraviolet (DUV) APSM fabrication. For 0.2 .mu.m contact holes, for example, insufficient depth of focus is achieved for only a 6% transmittance APSM. Depth of focus (DOF) refers to when the foreground is in focus while the background is out of focus, or visa versa. This occurs in normal photography as well with the wavelengths used in patterning microminuatized integrated circuits through photolithography. Even the micron and sub-micron sized topography on a wafer is sufficient to cause depth of focus problems.
U.S. Pat. No. 5,789,116 to Kim describes halftone phase-shift masks having a high transmittance that comprise a transparent substrate, a phase shifter pattern formed on the substrate and a phase shifting groove that can be formed by etching the transparent substrate.
U.S. Pat. No. 5,792,578 to Tzu et al. describes a method of forming a double layer attenuating phase-shifting mask with a first and second resist that avoid having to withstand dry etching by the use of a pellicle that restricts the deposition of the second resist to the alignment region of the mask. Cleaning agents such as H.sub.2 O.sub.2 +H.sub.2 SO.sub.4 followed by H.sub.2 O+NH.sub.4 OH+H.sub.2 O.sub.2 or the like may be used to thoroughly clean the mask after the layer of first resist is removed.
U.S. Pat. No. 5,503,951 to Flanders et al. describes an attenuating phase-shift, semiconductor fabrication mask having recessed attenuating and phase-shifting regions that is not susceptible to phase defects in the printing regions of the mask. The surfaces of the fully transmissive regions of the mask are not altered and the attenuating regions of the mask are recessed relative to the fully transmissive regions.
U.S. Pat. No. 5,618,643 to Dao et al. describes a method and apparatus for fabricating an attenuated phase-shifting mask for use in patterning a radiation sensitive layer in a lithographic printer. In one embodiment a layer of embedded phase shift material overlies a transparent base material and attenuates the radiation and changes the phase of transmitted radiation by amounts other than 160-200.degree.. Subsequent processing to the base material is performed to result in a net 180.degree. phase difference between radiation transmitted by the phase shift layer and the transparent base material. Such processing of the base material may be accomplished by subsequent etching of the quartz base.
U.S. Pat. No. 5,614,335 to Hashimoto et al. describes a halftone phase-shift photomask and a blank thereof which enables the transmittance of a phase shift portion to be varied between 1% to 50% even after blank or photomask fabrication. This is accomplished by exposing the blank or photomask to a high temperature elevated to at least 150.degree. C.; to an oxidizing atmosphere; or to a reducing atmosphere in a step that can be independent from the film-forming or photomask fabrication steps.
U.S. Pat. No. 5,667,919 to Tu et al. describes an APSM and a APSM blank each having a tantalum-silicon oxide blanket semitransparent shifter layer formed over a transparent substrate. The tantalum-silicon oxide having the formula: EQU Ti.sub.x Si.sub.y O.sub.1-x-y
where 0.10&lt;x&lt;0,3 and 0,03&lt;y&lt;0.1.
U.S. Pat. No. 5,601,954 to Krivokapic et al. describes a process of fabricating an attenuated phase-shifting mask having a first layer with a thickness to provide a 3 to 10% transmittance formed on a transparent substrate. A second layer is formed over the first layer and has a thickness to provide a desired phase shift. For a phase shift of 180.degree. and i-line wavelength (365 nm), chromium is used as the first layer having a thickness of from 25 to 75 nm and silicon dioxide is used as the second layer having a thickness of from 400 to 450 nm. The chromium and silicon dioxide layers are etched, preferably by an isotropic wet etch, to form parabolically shaped sidewalls in the silicon dioxide layer that more efficiently focuses the light through the opening in the thin chromium layer.
U.S. Pat. No. 5,869,212 to Hashimoto describes an attenuated phase-shifting mask employing regions of increased light transmissivity adjacent the defined circuit pattern features which improves image resolution and depth of focus, with a minimization of image shortening effects. These regions may be provided by partially oxidizing a secondary region of the halftone masking layer. In an alternate embodiment sharper corner definition may be also be obtained by providing a generally rounded, light diffracting topography at edges of the defined circuit pattern features. In the case of a line feature, the substrate can be etched to form a recessed region including a generally concave light transmissive surface extending beneath an edge portion of the masking layer.
U.S. Pat. No. 5,786,114 to Hashimoto describes an attenuated phase-shift mask having boundary regions formed from the halftone layer used to form the circuit pattern regions. The boundary regions are shielded from a stabilization treatment of the halftone layer serving to stabilize the transmittance of the circuit pattern regions at an increased optimum level. The transmittance of the boundary regions remains relatively low thus avoiding the problem of exposure light leaking to adjacent chip areas in a step and repeat or scanning exposure process. This process eliminates the extra steps required to form isolation frames and alignment marks from a separate opaque layer and avoids the need for data sensitive sub-resolution patterning.
U.S. Pat. No. 5,795,682 to Garza describes an attenuated phase-shift reticle, or mask, design having a compensating transmissive region located where side lobe ringing is anticipated to be most severe. These compensating transmissive regions, unlike other transmissive regions on the reticle, do not have defined integrated circuit features. The radiation transmitted through the compensating transmissive region is approximately 180.degree. out of phase with the radiation giving rise to side lobe ringing so as to destructively interfere with the out of phase radiation.